UNDERSTANDING HERBICIDE MODES OF ACTION AND APPLICATION METHODS IN CROPPING SYSTEMS

Transcription

1 UNDERSTANDING HERBICIDE MODES OF ACTION AND APPLICATION METHODS IN CROPPING SYSTEMS Case R. Medlin Weed Extension Specialist Department of Plant and Soil Science - Oklahoma State University Table of Contents I. Introduction to Herbicide Mode of Action Groups II. Herbicide Mode of Actions Classification Table III. Discussion of Herbicide Groups, Families, and Specific Herbicides

2 I. INTRODUCTION TO HERBICIDE MODE OF ACTION GROUPS To simplify discussion in this publication, herbicides have been divided into eleven major categories, according to activity. In addition to this classification system, these herbicide groups are subdivided according the Weed Science Society of America s herbicide classification standard. Like any other classification system, there are always some overlapping and exceptions. However, for all practical purposes, these categories should help the layperson better understand how herbicides function in the plant and how best to use them. It is important to understand the soil system along with its colloidal properties, the chemical impact of soil ph, and living components of the soil to understand how herbicides work in the soil and how this system is subjected to outside influences. It is also important to understand adsorption, movement and degradation of herbicides in the soil.

3 II. HERBICIDE MODE OF ACTION CLASSIFICATION TABLE MOA Group Site of Action & Description Family Trade Name Common Name I Lipid A - Inhibits ACCase Aryloxyphenoxy Assure II quizalofop Biosynthesis propionate Clincher cyhalofop Inhibitors Discover clodinafop Fusilade fluazifop Hoelon diclofop Puma fenoxaprop Cyclohexanedione Achieve tralkoxydim Envoy clethodim Poast sethoxydim Prism clethodim Select clethodim II Branched-chain B - Inhibits ALS (or AHAS) Imidazolinone Arsenal imazapyr Amino Acid Assert imazamethabenz Inhibitors Beyond imazamox Cadre imazapic Image imazaquin Plateau imazapic Pursuit imazethapyr Raptor imazamox Scepter imazaquin Pyrimidinylthio-benzoate Staple pyrithiobac Sulfonylamino- Everest flucarbazone carbonyltriazolinone Olympus propoxycarbazone Sulfonylurea Accent nicosulfuron Ally metsulfuron Amber triasulfuron Battalion halosulfuron Beacon primisulfuron Cimarron metsulfuron Classic chlorimuron Enfield trifloxysulfuron Equip foramsulfuron Escort metsulfuron Express tribenuron Glean chlorsulfuron Harmony thifensulfuron Logran triasulfuron Londax bensulfuron Maverick sulfosulfuron Muster ethametsulfuron Option foramsulfuron Oust sulfometuron Peak prosulfuron Permit halosulfuron Rave triasulfuron Sandea halosulfuron Safari triflusulfuron Telar chlorsulfuron Titus rimsulfuron Upbeet triflusulfuron Triazolopyrimidine Amplify cloransulam First Rate cloransulam Primus florasulam Python flumetsulam Strongarm diclosulam

4 III Classical C1 - Inhibits photosynthesis at Phenyl-carbamate Batamex desmedipham Photosynthesis photosystem II site A Betanal phenmedipham Inhibitors Spin-Aid phenmedipham Pyridazinone Pyramin pyrazon Triazine AAtrex atrazine Caparol prometryn Evik ametryn Pramitol prometon Princep simazine Velpar hexazinone Triazinone Lexone metribuzin Sencor metribuzin Uracil Hyvar X bromacil Sinbar terbacil IV Rapid-acting C2 - Inhibits photosynthesis at Amide Propanil propanil Photosynthesis photosystem II site A 2 Stam propanil Inhibitors Urea Cotoran fluometuron Direx diuron Karmex diuron Linex linuron Lorox linuron Meturon fluometuron Preflan tebuthiuron Spike tebuthiuron Tupersan siduron C3 - Inhibits photosynthesis at Benzothiadiazole Basagran bentazon photosystem II site B Nitrile Buctril bromoxynil Phenyl-pyridazine Tough pyridate V Cell Membrane D - Photosystem I electron Bipyridylium Cyclone paraquat Disruptors diverters Diquat diquat Gramoxone paraquat Reglone diquat E - Inhibits Protox Diphenylether Cobra lactofen Flexstar fomesafen Goal oxyfluorfen Reflex fomesafen Ultra Blazer acifluorfen N-phenylphthalimide Action fluthiacet Resource flumiclorac Valor flumioxazin Oxadiazole Ronstar oxadiazon Pyrimidindione Inspire butafenacil Triazinone Aim carfentrazone Teamwork carfentrazone Authority sulfentrazone Spartan sulfentrazone Triazolone Milestone azafenidin VI Pigment F1 - Inhibits PSD Pyridazinone Zorial norflurazon Inhibitors None Sonar fluridone F2 - Inhibits 4-HPPD Triketone Callisto mesotrione Isoxazole Balance isoxaflutole F3 - Target unknown Triazole Amitrol T amitrole F4 - Inhibits DOXP synthase Isoxazolidinone Command clomazone VII Inhibits Ringed G - Inhibits EPSP synthase None Many glyphosate Amino Acids Roundup glyphosate

5 VIII Seedling Root K1 - Inhibits microtubule Dinitroaniline Balan benefin Inhibitors assembly Barricade prodiamine Prowl pendimethalin Sonalan ethalfluralin Surflan oryzalin Treflan trifluralin Pyridine Dimension dithiopyr Visor thiazopyr Others Dacthal DCPA Betasan bensulide IX Seedling Shoot K3 - Inhibits very long-chain Acetamide Devrinol napropamide Inhibitors fatty acids Chloroacetamide Dual metolachlor Frontier dimethenamid Harness acetochlor Lasso alachlor Machete butachlor Outlook dimethenamid Pennant metolachlor Ramrod propachlor Surpass acetochlor TopNotch acetochlor Oxyacetamide Define flufenacet N - Inhibits lipid synthesis Thiocarbamate Arcade prosulfocarb Avadex triallate Bolero thiobencarb Boxer prosulfocarb Defi prosulfocarb Edge pebulate Eptam EPTC Eradicane EPTC Fargo triallate Genate+ butylate Ordram molinate Ro-Neet cycloate Tillam pebulate Vernam vernolate X Growth O - Synthetic auxins Benzoic acid Banvel dicamba Regulators Clarity dicamba Carboxylic acid Garlon triclopyr Reclaim clopyralid Starane fluroxypyr Stinger clopyralid Tordon picloram Phenoxy Many 2,4-D Many 2,4-DB Many 2,4-DP Many MCPA Many MCPB Many mecoprop/mcpp Quinoline Carboxcylic Drive quinclorac acid Paramount quinclorac XI Miscellaneous H - Inhibits glutamine synthase None Finale glufosinate Ignite glufosinate Liberty glufosinate L - Inhibits cell wall synthesis Benzamide Gallery isoxaben Z - Unknown Organoarsenical Many DSMA Many MSMA

6 I Lipid Biosynthesis Inhibitors A. Introduction These herbicides were developed for selective postemergence grass control in broadleaf crops. These compounds are absorbed readily by plant tissue and move in the symplast to growing points where they inhibit meristematic activity in the roots and shoots. Good growing conditions of the plants at time of application are critical for adequate weed control. B. WSSA Herbicide MOA Classification (A) Inhibit ACCase ACCase is an enzyme used in the biosynthesis of fatty acids, which are used in the makeup of lipids. These lipids are used in a variety of cell components including cell and organelle membranes. Since these herbicides inhibit the further production of lipids (and do not destroy lipids that already in functioning cells) their activities are first seen in the meristematic regions of plants. C. Herbicide Families Aryloxyphenoxy propionate & Cyclohexanedione; AKA fops & dims D. Symptoms The first effect on the shoot is death of the growing point and the inner whorl of leaves. Grasses stop growing and turn red to purple in color several days after herbicide application (Figure 1). In johnsongrass a few days after application one can pull back the outer leaves at the nodes and see the dark colored decaying tissue. Five to seven days after application, stems of treated plants can be pulled apart at the nodes, this is generally referred to as stem slippage (Figure 2). Figure 1. Figure 2. E. Specific Herbicides 1. The Aryloxyphenoxy propionates the Fops Assure II (quizalofop - ethyl) is one of the newer herbicides in this group that has been approved for use in several broadleaf crops. A yellow flash on leaves of broadleaf crops can occur but is attributed to the solvent system in the herbicide formulation and does not negatively impact crop growth. Discover (clodinafop) is labeled in wheat for postemergence control of wild oats. Hoelon (diclofop - methyl) was the first of this group. In Oklahoma, it is used to control Italian ryegrass and wild oat in wheat. It is not widely used in Oklahoma due to its full-season grazing restriction following its application.

7 Fusilade (fluazifop - butyl) is another herbicide in this group that has been developed for several years and is approved for use in many different broadleaf crops for annual and perennial grass control. It is also labeled for control of annual and perennial grasses (e.g. bermudagrass) in Liriope species (i.e. monkey grass). Puma (fenoxaprop - ethyl) has the same active ingredient as several other herbicides, including Whip, Bugle, and Silverado. Puma has replaced Silverado in Oklahoma for control of wild oats in winter wheat. Puma contains a safener that reduces the injury potential on the wheat crop. 2. The Cyclohexanediones Dims Achieve (tralkoxydim) is used for postemergence control of wild oats and Italian ryegrass in winter wheat. Envoy (clethodim) is labeled in various ornamental and tree crops for postemergence control of annual and perennial grasses. Poast (sethoxydim) controls both annual and perennial grasses and is approved for use in many agronomic, broadleaf crops, horticultural crops and landscape plantings. Prism & Select (clethodim) controls both annual and perennial grasses and is approved for use in many agronomic broadleaf crops and horticultural crops. II Branched-chain Amino Acid Inhibitors A. Introduction These herbicides have wide diversity in plant tolerance and weed control purpose. Although there are several herbicide families within this mode of action group, the sulfonylurea and imidazolinone families were the first discovered and marketed. Members of the ALS inhibiting families can be used on wheat that would rapidly kill soybeans while other herbicides in the family are approved for use in soybeans, corn, cotton, turfgrass, and even vegetable crops. Rates are extremely low at less than an ounce per acre. They are absorbed by both shoots and roots, and are readily translocated within the plant. They immediately stop growth of susceptible weeds and the plants gradually decline in vigor. We often see a reddish-purplish color in affected plants before they die. Carryover duration of the sulfonyureas ranges from 1 month to 3 years. Their persistence in the soil is highly dependent on soil moisture and soil ph. Under high soil ph conditions (ph > 7) and low precipitation, many of the sulfonylurea herbicides can persist for several years. Crop safety can be jeopardized when the Sulfonylureas are applied in conjunction with, or within a few days of an OP insecticide, see label for details. B. WSSA Herbicide MOA Classification (B) Inhibit ALS or AHAS These herbicides inhibit plant growth by binding to the ALS (acetolactate synthase) enzyme which is critical for the production of three essential, branched-chain amino acids. Elimination of these amino acids from the plant s meristematic region results in the loss of essential proteins that contain those amino acids. Without these essential proteins, the plant is unable to grow. The blockage of the branched-chain amino acid pathway also leads to a buildup of substrates that eventually reaches toxic levels.

8 These herbicides prevent the production of three amino acids and do not destroy the amino acids already composing proteins. For this reason, it often takes several days or even a week before visual symptoms can be seen. C. Herbicide Families Imidazolinones, Pyrimidinylthio-benzoate, Sulfonylaminocarbonyltriazolinone, Sulfonylurea, and Triazolopyrimidine D. Symptoms Initial symptoms can appear as early as 3 to 4 days after treatment, but usually do not occur for 7 to 10 days after treatment. Initial symptoms include stunting and interveinal chlorosis (Figure 1) of the growing point. As symptoms progress, internode length of plants becomes shorter, and chlorosis turns to necrosis. Secondary symptoms that may be seen include dark red to black veins on some broadleaf plants (including soybean), purple stems (Figure 2) on some grasses (including corn and sorghum), bottlebrush roots (Figure 3) on susceptible grasses (especially corn and sorghum during instances of herbicide carryover). Figure 1 Figure 2 Figure 3 (Untreated on left) E. Specific Herbicides 1. The Imidazolinones Arsenal (imazapyr) is a non-selective herbicide for use on noncropland. It will control a broad spectrum of both grasses and broadleaf weeds, including field bindweed and some tree species. It is also one of the active ingredients in Lightning, the broad-spectrum herbicide labeled for postemergence use in Clearfield corn varieties. Beyond and Raptor (imazamox) are labeled for use in a variety of different crops. Beyond is labeled for winter annual grass and broadleaf control in Clearfield wheat cultivars. Raptor is labeled for postemergence control of seedling grasses and annual broadleaf weeds in soybeans, and summer and winter annual weeds in alfalfa. Cadre (imazapic) controls broadleaves and small seedling grasses in peanuts. Cadre can be applied preemergence or postemergence in peanuts, but has more activity when applied postemergence. Pursuit (imazethapyr) is similar to Scepter, but is effective on several more weeds when applied postemergence. Pursuit is labeled for use in soybean, peanuts, alfalfa, and Clearfield rice and corn.

9 Scepter and Image (imazaquin) are excellent herbicides for broadleaf weed control in soybeans (Scepter) and turfgrass (Image). These products are used primarily preplant or preemergence, but do have postemergence activity. They will control cocklebur and pigweeds early postemergence but do not control a broad spectrum of weeds as they do if soil-applied. Scepter does have a long residual in the soil and can cause carryover problems to rotational crops. Scepter is often mixed with a Dinitroaniline to increase its control of grasses. 2. Pyrimidinylthio-benzoate Staple (pyrithiobac) is labeled for preemergence or over-the-top, broadleaf weed control in cotton. It is often viewed as the first selective, postemergence, broadleaf herbicide used in cotton. 3. Sulfonylamino-carbonyltriazolinone Everest (flucarbazone) is labeled for postemergence control of cheat and Japanese brome in winter wheat. Olympus (propoxycarbazone) is labeled for postemergence control of cheat in winter wheat. 4. The Sulfonylureas (SUs) Accent (nicosulfuron) is used as a selective postemergence control of annual and perennial grasses and broadleaves in corn. The timing and application method of OP insecticides applied at planting or postemergence to the corn crop is critical to crop sensitivity, refer to label for specifics. Ally, Cimarron, and Escort (metsulfuron methyl). Ally is labeled for use in wheat and sorghum for postemergence control of broadleaf weeds. In sorghum, Ally can be tank mixed with 2,4-D to increase the spectrum and speed of activity of both herbicides. Cimarron is used in roadsides, rightof-ways, pastures, and rangelands for control of broadleaf weeds. When applied alone, the weed spectrum is somewhat narrow, but when applied with 2,4-D, 2,4-D + picloram, or 2,4-D + dicamba the spectrum of activity increases greatly. Escort is labeled for non-cropland use and for use in conifer plantations. Beacon (primisulfuron) is used for seedling grass and broadleaf weed control in corn. Although it has substantial grass activity, it is a better broadleaf herbicide than grass herbicide. Classic (chlorimuron) is used for postemergence broadleaf weed control in soybean. It can also be used preemergence but is usually applied with metribuzin or sulfentrazone in such an application. Glean and Telar (chlorsulfuron) are used to control broadleaf weeds in small grains (Glean) and turfgrass (Telar). These products will also control Italian ryegrass preemergence or early postemergence under specific environmental conditions. Glean may remain in the soil several months at levels that will kill some rotational crops. Harmony GT (thifensulfuron) is a short residual herbicide labeled for use in wheat and as a tank mix for preplant burndown applications. It will remain in the soil long enough to provide good weed control, but not carryover to succeeding crops. It is primarily taken-up by the foliage of plants, so weeds must be emerged and actively growing for this herbicide to be effective. Maverick (sulfosulfuron) was the first selective, postemergence cheat control herbicide labeled for use in wheat. It has fair to good activity on rescuegrass and downy brome when these weeds are treated in the 1-2 leaf stage of development.

10 Muster (ethametsulfuron) is labeled to control broadleaf weeds in canola. Option (foramsulfuron) is used for annual and perennial grass and broadleaf weeds in field corn. Oust (sulfometuron) controls many annual and perennial weeds on noncropland areas. It can be used at low rates on roadsides and other areas where desirable perennial grasses grow to kill the weeds and release the grasses such as bermudagrass and bahiagrass. Permit, Battalion, and Sandea (halosulfuron) are labeled for use in field corn (Permit), sweet corn (Sandea), and grain sorghum (Permit). Their strengths are common lambsquarters and nutsedge control. 5. Triazolopyrimidine Amplify, First Rate (cloransulam) are labeled for broadleaf weed control in soybeans. They are labeled for preemergence and postemergence applications, but are generally used postemergence. These products have excellent activity on giant ragweed. Python (flumetsulam) is labeled for preemergence broadleaf weed control in field corn. Strongarm (diclosulam) is labeled for preemergence applications in peanuts. Strongarm has excellent activity on eclipta, one of the worst weed problems in Oklahoma peanut growing areas. III Classical Photosynthesis Inhibitors A. Introduction The Classical Photosynthesis Inhibitors is an important group of herbicides in modern day agriculture. This group is composed of herbicides such as atrazine, metribuzin, simazine, and hexazinone, some widely used herbicides in Oklahoma. These products have wide spectrums of activity and are tankmixed with other products to further their weed control abilities. B. WSSA Herbicide MOA Classification (C1) Inhibit photosynthesis at photosystem II site A These herbicides interfere with the passage of electrons through the photosystem II complex. Specifically, these herbicides bind to the D1 protein in photosystem II. Blocking the electron chain results in the production of free radicals in the chloroplasts, which results in cellular membrane degradation. C. Herbicide Families Phenyl-carbamate, Pyridazinone, Triazine, Triazinone, and Uracil D. Symptoms These herbicides are dependent on the photosynthesis process for activation, therefore, when these herbicides are applied preplant incorporated or preemergence, plants must emerge from the soil and begin photosynthesizing before symptoms will occur. These herbicides are readily translocated via the xylem in plants. Since fully functioning, mature leaves are the greatest sink for the xylem, symptoms first appear on lower portions of the plant as chlorosis of leaf margins on broadleaf plants and chlorosis of leaf tips of grasses. This is generally followed by necrosis of the leaf margins and chlorosis between the veins of broadleaf plants (Figure 1) and a gradual chlorosis followed by necrosis from the tips to the collars of grass leaves (Figure 2).

11 Figure 1 Figure 2 E. Specific Herbicides 1. Phenyl-carbamates Spin - Aid (phenmedipham) is a useful herbicide in spinach and beets where the number of alternatives is very limited. 2. Triazines AAtrex (atrazine) is used in many crops. It is a major herbicide in sorghum and corn and is used extensively in postemergence applications early in the spring in roadside and lawn applications in Oklahoma. It sells as AAtrex, Atrazine, and under several other trade names. There are leaching concerns with atrazine. It causes more carryover and injury problems on high ph soils. Caparol (prometryn) is selective for annual weed control in cotton. preemergence and directed postemergence on medium and fine textured soils. It is used preplant, Pramitol (prometon) is a nonselective preemergence and postemergence herbicide. It primarily works through the soil. It may persist several seasons and is a good soil sterilant. Princep (Simazine) is heavily used in nurseries, Christmas trees, fruit and nut crops and herbaceous perennial crops such as asparagus. It must be applied before weed emergence. Velpar (hexazinone) is used primarily in alfalfa, pine release, brush control and non-cropland. 3. Triazinones Sencor (metribuzin) is selective for control of annual weeds in soybeans, wheat (some varieties), potatoes, tomatoes, asparagus and alfalfa. It is used preemergence in soybean, but cannot be applied to wheat until it has tillered and formed a root system. 4. Uracils Sinbar (terbacil) controls a wide variety of annual weeds and several perennial weeds. Its main uses are in alfalfa, asparagus, apples, and peaches in Oklahoma.

12 Hyvar X (bromacil) is a long residual non-selective herbicide used for general vegetation control. It is one of our two basic soil sterilants. It should be applied before weeds come up. The half-life is 5 to 6 months. It sells in many different formulations and mixtures, including Hyvar, Krovar, Ureabor, etc. IV Rapid-acting Photosynthesis Inhibitors A. Introduction The Rapid-acting Photosynthesis Inhibitors are generally viewed as the postemergence herbicides that are available to cleanup any escaped weed problems in fields. Without these herbicides, many weed escapes would be hand rouged from fields. B. WSSA Herbicide MOA Classification (C2) Inhibit photosynthesis at photosystem II site A 2 (a different binding site than the C1 group) or (C3) Inhibit photosynthesis at photosystem II site B. The Ureas are readily absorbed by plant roots and translocated to the shoots through the apoplast. They are photosynthesis inhibitors (very similar to Group III above but quicker acting). Light is required for herbicidal activity. Selectivity primarily depends on placement or differential translocation and metabolism. These herbicides interfere with the passage of electrons through the photosystem II complex just as Group III, but have a slightly different binding site. C. Herbicide Families Amide, Urea, Benzothiadiazole, Nitrile, and Phenyl-pyridazine D. Symptoms These herbicides are dependent on the photosynthesis process for activation, therefore, when these herbicides are applied preplant incorporated or preemergence, plants must emerge from the soil and begin photosynthesizing before symptoms will occur. These herbicides are readily translocated via the xylem in plants. Although speed of activity is generally much greater, many of the injury symptoms are similar to the Triazines, first showing interveinal and margin chlorosis followed by necrotic tissue and dying, starting at the tips and leaf margins of mature leaves (refer to Figures 1 & 2 above). Some of these, however, show veinal chlorosis instead of interveinal. An example of this is diuron injury on cotton or grapes. Fluometuron can cause total chlorosis of the interveinal spaces on some lower cotton leaves (Figure 1) without necrosis occurring. Although labeled for use in corn, Buctril will cause significant burning of contacted leaf tissue (Figure 2). Figure 1 Figure 2

13 E. Specific Herbicides 1. Amides Propanil or Stam (propanil) is used to control many grasses and broadleaf weeds in rice production. 2. Ureas Cotoran (fluometuron) is used preemergence and postemergence in cotton. It is formulated under other trade names in addition to Cotoran, such as Meturon. Direx or Karmex (diuron) is used in cotton, alfalfa, woody ornamentals, and fruit and nut crops. It requires a surfactant when used postemergence. It is also used in abatement programs where complete soil sterilization is desired. Spike (tebuthiuron) is a long lasting urea herbicide for general vegetation control in non cropland and for brush control in rangeland. It is very water soluble and can readily leach through sandy soils. Tupersan (siduron) is different from other Ureas. It does not affect photosynthesis, but inhibits root growth of weed seedlings. It is effective for control of annual grasses in established turf grasses, including turf grasses such as fescue, perennial ryegrass and Zoysia. 3. Benzothiadiazoles Basagran (bentazon) is a contact herbicide used postemergence for the selective control of many broadleaf weeds and sedges in grass crops, turfgrass, and large seed legume crops such as soybeans and peanuts. It inhibits photosynthesis, but translocation is limited, therefore, thorough coverage of weed foliage is necessary. It is used in Oklahoma on soybeans, peanuts, corn, sorghum, peas, beans and turf. It is good for control of cocklebur and yellow nutsedge along with several other weeds. 4. Nitriles Buctril (bromoxynil) is sold in mixtures such as Bronate. It is used primarily to control seedling broadleaf weeds in grass type crops; corn, sorghum and small grains. The big use in Oklahoma is in wheat in areas where specific weed problems such as wild buckwheat exists or where drift and carryover of hormone type herbicides, such as 2,4-D or Banvel, may be a problem. It will cause quick burns on leaves of labeled crops. V Cell Membrane Disruptors A. Introduction The cell membrane disruptors are a classification of herbicides that kill primarily by contact action. This herbicide group contains herbicide families with two very different modes of action, however, their end results (i.e. the destruction of cellular membranes), their symptomology (i.e. total necrosis of plant foliage contacted with the spray) and their speed of kill (i.e. very quick) are similar. For these reasons, these herbicides will be discusses together. These herbicides are used in a wide variety of situations ranging from preplant burndown of all vegetation to selective over-the-top applications in various crops. Some of the major crops these herbicides are labeled in include soybeans, peanuts, wheat, sorghum, and sunflower. One herbicide, diquat, even has aquatic uses.

14 B. WSSA Herbicide MOA Classification (D) Photosynthesis I electron diverters or (E) Inhibit protox The Bipyridylium divert electrons from the photosynthesis pathway and create very harmful free radicals that disrupt cellular membranes. The Diphenylethers, N-phenylphthalimides, Oxadiazoles, Pyrimidindiones, Triazinones, and the Triazolones inhibit protoporphyrinogen oxidase (PROTOX), an enzyme needed in the development of chlorophyll. This blockage not only prevents further production of chlorophyll, but also results in buildup of energy absorbing compounds that become photo synthesizers when excited by the absorbed light energy. These energy rich compounds then produce free radicals that quickly destroy cellular membranes. C. Herbicide Families Bipyridylium, Diphenylether, N-phenylphthalimide, Oxadiazole, Pyrimidindione, Triazinone, and Triazolone D. Symptoms These herbicides range in use from non-selective, total postemergence products to selective preemergence products. One thing that is constant among these products is the requirement of light to begin the cell destruction process, therefore, when these herbicides are applied to the soil, emergence of the weed must occur before the herbicide is activated. For postemergence products, spray coverage is critical (these products are typically applied with at least 15 gallons per acre carrier volume) for complete kill of emerged weeds. Paraquat kills leaf tissue so fast that individual drift droplets cause white necrotic spots on leaves of plants within a few hours after application (Figure 1). Most of these herbicides are used as postemergence sprays and have little or no soil activity, but fomesafen (Reflex or Flexstar) has a 10 month rotation restriction for planting corn due to its ability to persist in the soil and cause veinal chlorosis of the next year s corn crop (Figure 2). Lactofen (Cobra), fomesafen (Reflex or Flexstar), and acifluorfen (Ultra Blazer) can all cause leaf burn of soybeans or peanuts (Figure 3). Figure 1 Figure 2 Figure 3 E. Specific Herbicides 1. Bipyridyliums - These herbicides are not selective. They are membrane disruptors. Activity of these herbicides depends on light, therefore, maximum performance could be expected when temperature is high and the sun is shining. They are cations and adsorb quickly and tightly to the soil, which means one can plant a crop after use of these herbicides without expecting injury. These herbicides are extremely toxic to mammals when ingested.

15 Gramoxone Extra (paraquat) is used with a surfactant as a nonselective preplant burndown treatment. It burns off seedling weeds rapidly. Larger weeds may send up new shoots from established roots and perennials are usually not controlled. Paraquat may be used to kill weeds in a crop field before planting, as a directed spray at low rates on certain crops (corn and peanuts), and as a harvest aid to desiccate weeds and crop leaves for harvest (cotton). Diquat is used in some crops as a preharvest aid, in turf to control weeds when grass is dormant, in turf renovation, and in aquatic weed control. 2. Diphenylethers Cobra (lactofen) is approved for use in soybeans. The spectrum of weeds it controls is broader than most of the other herbicides in this family, but it is the most injurious herbicide of the family. Near complete desiccation of the soybean plant can be expected after Cobra has been applied, but only minor, if any, soybean yield reduction usually results. Flexstar or Reflex (fomesafen) is slightly less active on weeds than Cobra, but is much safer on the soybean crop. It has more soil residual activity, and does have a 10 month rotation restriction for planting back to corn. It will cause veinal chlorosis in the corn if it is planted back too quickly following a Flexstar or Reflex application. Goal (oxyfluorfen) is used in Christmas trees, nurseries, tree fruits and as a directed spray in cotton. It is used preemergence in some of these crops. Ultra Blazer (acifluorfen) is highly used in soybeans and peanuts for control of weeds such as pigweeds, spurges, crotons, copperleaf and morningglory. If one of the three weeds which Basagran is most effective for controlling is present along with these weeds, a mixture of Basagran and Blazer is often used [See weeds under Basagran above]. Mixtures on the market containing Basagran and acifluorfen include Storm and Galaxy. 3. Oxadiazoles Ronstar (oxadiazon) is effective for controlling annual weeds in ornamentals and turf. Ronstar affects seedlings as they emerge from the soil, therefore, it is used preemergence to weed germination. Its half life is from 3 to 6 months. 4. Triazinone Aim or Teamwork (carfentrazone) is approved for use in wheat, corn, cotton, soybeans, and sorghum. It has a fairly narrow spectrum of weeds that it totally controls, but it can be tank-mixed with several other herbicides to increase their spectrums of activity. When applied alone, it is very effective for control of velvetleaf and morningglory. Authority or Spartan (sulfentrazone) is labeled for preemergence weed control in soybeans, sunflower, and tobacco. In soybeans it is prepackaged with chlorimuron (Classic) and sold as Canopy XL. Injury symptoms on many species include random black spots across leaf surfaces.

16 VI Pigment Inhibitors A. Introduction Key herbicides commonly used in cotton, corn, and soybean production are grouped within the pigment inhibitors. Most of these herbicides are soil-applied and are intended to control small-seeded broadleaf weeds and grasses. Exceptions include mesotrione (Callisto) and amitrole (Amitrole T). These herbicides can be applied postemergence and amitrole is used for controlling perennial vines. B. WSSA Herbicide MOA Classification (F1) Inhibit PSD; (F2) Inhibits 4-HPPD; (F3) Target Unknown; (F4) Inhibits DOXP synthase These herbicides inhibit the production of carotenoid pigments. Carotenoid pigments are used to calm free radials to a safe state in the plant. These free radicals include triplet chlorophyll in the photosynthesis process. By inhibiting the production of these carotenoid pigments, these herbicides prevent the triplet chlorophyll from being calmed to a safe state in the chloroplasts, and the result is the production of many free radicals that cause the degradation of cellular membranes. Perhaps the first cellular membrane to burst is the chloroplast membrane. This results in the loss of functioning chlorophyll in the plant and thus a bleached white appearance. C. Herbicide Families Pyridazinone, Triketone, Isoxazole, Triazole, Isoxazolidinone, and other herbicides D. Symptoms Since these herbicides are carotenoid pigment inhibitors they block the production of new carotenoid pigments but do not actively destroy the carotenoid pigments already in the plants. For this reason, the initial symptoms of these herbicides appear as white foliage on the new growth. However, as a plant matures functioning carotenoid pigments must be replaced (even in the old growth of plants), thus white foliage will soon appear even in the old growth of plants. A topical or drift application of these herbicides to well established yet developing plants will likely result in plants with green actively growing lower parts and yellow to white new growth (Figures 1 and 2). Figure 1. Figure 2. E. Specific Herbicides 1. Pyridazinones Zorial (norflurazon) is used preemergence as a selective herbicide in cotton. amounts in soybeans or other broadleaf crops causes veinal chlorosis and necrosis. Small carryover

17 2. Triketone Callisto (mesotrione) is used preemergence and postemergence in field corn to control annual broadleaf weeds. Balance PRO (isoxaflutole) is used preemergence in field corn to control annual broadleaf and grassy weeds. Its weaknesses include morningglory species and johnsongrass. 3. Triazole Amitrol T (amitrole) is a non-selective herbicide that causes whitening of the new tissue formed after application. It is often used to control certain woody plants and vines in sensitive areas where growth regulator type herbicides cannot be used. It is also used in nursery stock, ornamental plantings and forest plantations. 4. Isoxazolidinone Command (clomazone) is good for control of annual grasses and velvetleaf in soybeans, peas and chemical fallow. It can also be soil-applied in cotton, but only if an Organo-phosphate insecticide has been applied in-furrow. VII Ringed Amino Acid Inhibitors A. Introduction Glyphosate has become a very important part of modern agriculture crop production since the advent of glyphosate tolerant crops through biotechnology. These Biotech Crops, including Roundup Ready corn, soybeans, and canola, are resistant to glyphosate and will tolerate over-the-top applications of the herbicide. Prior to their use in modern day agriculture, glyphosate was used solely as a preplant burndown, harvest-aid, and a non-cropland herbicide. Glyphosate is absorbed by shoots of plants and has no soil activity since it is quickly degraded by soil microbes. It readily translocates within the plant, thus enabling it to control perennial plant structures beneath the ground. It immediately stops growth of susceptible weeds and the plants gradually decline in vigor. Glyphosate activity can be reduced when hard water is used as the carrier. To overcome this affect, ammonium sulfate (AMS), at 17 lb/100 gallons, should be mixed with the water prior to adding the glyphosate. Also, since numerous glyphosate formulations are currently available, one should carefully read the label to determine the appropriate additives/surfactants needed. B. WSSA Herbicide MOA Classification (G) Inhibit EPSP synthase Glyphosate inhibits plant growth by binding to the EPSP synthase enzyme which is critical for the production of three essential, aromatic (i.e. ringed) amino acids. Elimination of these amino acids from the plant s meristematic region results in the loss of essential proteins that contain those amino acids. Without these essential proteins, the plant is unable to grow. The blockage of the aromatic amino acid pathway also leads to a buildup of substrates (shikimate) that eventually reaches toxic levels. These herbicides prevent the production of three amino acids and do not destroy the amino acids already composing proteins. For this reason, it often takes one week to 10 days before visual symptoms can be seen.

18 C. Herbicide Families By WSSA criteria, glyphosate is not classified into a herbicide family, but it is sometimes referred to as a Phosphono Amino Acid D. Symptoms Glyphosate is only taken up by plant foliage and is readily translocated. It controls many grasses and some broadleaf weeds, and would generally be considered non-selective (except for Glyphosate tolerant crops). Initial symptoms can appear as early as 5 to 7 days after treatment, but usually do not occur for 7 to 10 days after treatment. Initial symptoms include stunting and chlorosis of the growing point (Figure 1). This is followed by necrosis of the same leaf tissue and eventually plant death. Drift rates often result in stunting of auxiliary bud growth and stacking of immature leaves/nodes (Figure 2). Some bleaching of tissue may occur as the symptoms progress. The mottling of leaves in fruit trees and ornamentals can often be observed from spray drift. Glyphosate is used in numerous ways around the home and often is sold under trade names other than Roundup for homeowners. Figure 1. Figure 2. E. Specific Herbicides 1. None Roundup (glyphosate) is used in cropland prior to planting crops or in fallow periods. It is also used in rope wicks and other ways of selective placement in susceptible crops. In glyphosate-tolerant crops it is used as an over-the-top spray. VIII Seedling Root Inhibitors A. Introduction These herbicide families are very important in Oklahoma row crop agriculture. The Dinitroanilines are by far the most important family in this group. Dinitroaniline herbicides are used in a wide range of broadleaf crops in Oklahoma, including alfalfa, soybeans, peanuts, cotton, mungbeans, many vegetable, fruit, and ornamental crops. They are also a major contender in the turf and lawn markets. They control annual grasses very effectively and many small seeded broadleaf weeds. Selectivity in most large-seeded crops can be achieved by planting the seed below the herbicide treated layer. Read the label to determine planting depth of crop when planning preemergence or preplant incorporated applications of these herbicides. Most of the herbicides in this group are incorporated

19 into the soil. Leaching is not a problem because of the tight adsorption to soil colloids and low solubility in water. B. WSSA Herbicide MOA Classification (K1) Inhibit microtubule assembly These herbicides inhibit the formation of microtubule assembly, which is a structure in newly dividing cells. When a cell replicates it s DNA (i.e. chromosomes) and begins the cell division process microtubules assist by separating the replicated chromosomes so that each new cell will have a complete set of chromosomes. However, when treated with one of these herbicides, the cell cannot form the microtubule assembly, cannot divide the chromosomes, and therefore ends up with multiple copies of the DNA material in a swollen, non-functioning cell. When many of these swollen cells exist at the root tip, the seedling cannot take up nutrients or water and dies from this. C. Herbicide Families Dinitroaniline, Pyridine, and Other Herbicides D. Symptoms Initial symptoms of these herbicides can be found primarily below the soil surface. They produce swelling of the root tips and the inhibition of the formation of lateral and secondary roots (Figure 1). The underground portion of the stem of injured plants will be thickened and shortened. The Dinitroaniline herbicides can cause the hypocotyls of broadleaf crops to swell or thicken and become brittle (Figure 2). These symptoms are more likely to occur during periods of cool wet conditions. This condition in soybean is called, Brittle Bean Syndrome, and when followed by a high wind, can result in stem breakage near the ground surface. Symptoms sometimes resemble phosphorus deficiency (i.e. red to purple stem) because of inadequate root hairs to take up phosphorus. Figure 1. (Untreated on right) Figure 2. E. Specific Herbicides 1. Dinitroanilines Balan (benefin) is labeled for preplant incorporated applications during the establishment phase of alfalfa. Barricade (prodiamine) is similar to pendimethalin but has longer residual activity. It is used in some turfgrass markets. It should not be used in areas where the sod has not completely filled as root growth of most turfgrass types will be slowed.

20 Prowl (pendimethalin) is a less volatile Dinitroaniline than Treflan, and is available for many of the same crops. It should be incorporated for most uses, but can be used preemergence in some situations if rainfall or irrigation follows application. This herbicide is sold as Pendulum and Pendimethalin in some turf markets. Sonalan (ethalfluralin) is used in several crops, including cotton, soybeans, peanuts, beans and peas. Surflan (oryzalin) is much less volatile than the other Dinitroanilines and is used in many fruit crops, several nut crops, and in container grown ornamentals. It is also used in Christmas trees and in minimum tillage fallow programs. It is more leachable than trifluralin. Treflan (trifluralin) is widely used in many broadleaf crops, including several of the vegetable crops. It is also used in turf, ornamentals and fruit crops. Soil incorporation of trifluralin is required. 2. Other Herbicides Dacthal is a soil applied herbicide that is probably approved for use in more crops than any other herbicide. It is taken up from soil and not by foliage. It must be in the soil and activated when the weed seeds germinate. This makes it very selective for crops that have emerged if weeds have not yet germinated. It is a mitotic inhibitor that inhibits growth of both root and shoots. One of the big uses is in turf production and home lawns for crabgrass control. It is also very selective for use in many vegetable crops, small fruits and ornamentals. In many of these small seeded crops, the number of herbicides available is very limited. Dacthal is good around the home for use in the lawn, flower beds, and many of the garden crops. Betasan (bensulide) is registered for use in turf and lawns. It should be applied in the early spring before summer annuals germinate or in the early fall before winter annuals germinate. It controls annual grasses and a few small seeded broadleaf weeds as they germinate. It should be watered in soon after application. It is sold in several formulations, including granules and fertilizer mixtures. IX Seedling Shoot Inhibitors A. Introduction The Seedling Shoot Inhibitors are composed of four herbicide families that have two different modes of action. However, all of the herbicides within these families inhibit shoot growth of seedling weeds. The Acetamides, Chloroacetamides, and Oxyacetamides affect shoot meristems by inhibiting leaf emergence from the coleoptiles of grasses, thus causing malformed leaves. These herbicides have some postemergence activity when used on tiny seedlings, but are generally considered to be ineffective as postemergence herbicides and are used as preplant incorporated or preemergence herbicides. They are used in many crops in Oklahoma, including corn, soybean, cotton, sorghum, and peanuts. The Thiocarbamates are used in Oklahoma primarily in alfalfa, corn, ornamentals, beets and spinach. They are shoot growth inhibitors, and appear to prevent elongation. They have high vapor pressures which make them very volatile, therefore, they must be incorporated into the soil immediately following application. These herbicides are readily degraded in the soil. The expected soil life is only 2 to 6 weeks, therefore, they should be applied and incorporated into the soil just prior to planting. Recent research has shown that soils can be conditioned to Thiocarbamates with repeated use, and

21 very rapid breakdown can occur in these soils. enhanced biodegradation. This phenomenon is sometimes referred to as B. WSSA Herbicide MOA Classification (K3) Inhibit very long-chain fatty acids The Acetamides, Chloroacetamides, and Oxyacetamides inhibit the synthesis of very long-chain fatty acids. These herbicides are also believed to interfere with mitosis and other biological systems in the plant. (N) Inhibit lipid synthesis The Thiocarbamates inhibit lipid production in plant cells. In particular, these herbicides inhibit the production of cuticular waxes that typically coat leaf surfaces. Without these waxes, affected plants over-transpire and are more susceptible to seedling diseases. C. Herbicide Families Acetamides, Chloroacetamides, Oxyacetamides, and Thiocarbamates D. Symptoms The Acetamides, Chloroacetamides, and Oxyacetamides cause very rough, crinkled leaves on susceptible broadleaf plants. Restricted growth of the margins causes some cupping and a wedgeshape of dicot leaves, (can cause heart-shaped leaves on soybeans) (Figure 1). In grass crops, the shoot may be malformed and the leaves rolled tighter than normal, this is often called buggy-whipping (Figure 2). Grain sorghum must have been treated with Concept or Screen seed treatment prior to planting, or these herbicides will cause significant crop injury to the crop. Thiocarbamates may cause abnormal development of plant leaves, even on tolerant crops. Internodes may be shortened. Typical symptoms include leaves that are sealed together; the surfaces are puckered and crinkled, and their margins are cupped. In corn or sorghum, shoots appear twisted and leaves tightly rolled. Expanding leaves may emerge in an abnormal manner and rupture the first leaves causing the stalk to shred (Figure 2). Figure 1. Figure 2.

22 E. Specific Herbicides 1. Acetamides Devrinol (napropamide) is an excellent choice for grass control and will also control many small seed broadleaf weeds in several fruit and nut crops, including strawberries and grapes, and several vegetable crops including cole crops, eggplants, melons, peppers and tomatoes. This herbicide should be incorporated or irrigated into the soil soon after application. 2. Chloroacetamides Dual (metolachlor) is somewhat similar to Lasso in spectrum of weeds it will control and crops it is used in. It appears to be slightly better for yellow nutsedge control than Lasso, but may be a little less effective than Lasso for control of some broadleaf weeds. The half life in the soil is a little longer than Lasso. It is considered to be a preemergence type herbicide, but is widely used in Oklahoma as a PPI application. It is prepackaged with atrazine and sold as Bicep II Magnum for use in corn and sorghum. Frontier or Outlook (dimethenamid) has a similar weed control spectrum as metolachlor (Dual) and acetochlor (Harness). It is labeled for use in soybeans, peanuts, corn and grain sorghum. Grain sorghum seed must have been safened with Concept or Screen prior to planting or significant crop injury will result. Harness or Surpass (acetochlor) has a little broader weed control spectrum than metolachlor (Dual) or dimethenamid (Outlook), however, it has a little higher potential to injure corn than the others. It is prepackaged with atrazine and sold as many different trade names including Harness Xtra, Surpass 100, and others. Lasso (alachlor) is used in many crops, including corn, cotton, soybeans, peanuts, dry beans, potatoes, grain sorghum (with Concept or Screen treated seed) and woody ornamentals. It controls annual grasses and some small seed broadleaf weeds as they germinate. It is primarily considered a preemergence type of herbicide, but it is also used PPI in some situations, especially for yellow nutsedge suppression. 3. Oxyacetamides Define (flufenacet) is somewhat similar to alachlor (Lasso) in spectrum of weeds it will control. It controls grasses and small-seeded broadleaf weeds in corn. It is prepackaged with metribuzin and labeled in soybeans as Domain and Axiom, and prepackaged with metribuzin and atrazine and labeled in corn as Axiom AT. 4. Thiocarbamates Eptam (EPTC) is probably the most widely used of the thiocarbamates. It is usually applied in corn in a formulation sold as Eradicane. Extenders can be added in the formulations to make it last a little longer in the soil. Eptam is an important herbicide in many ornamentals and is used in alfalfa and small seeded legumes, green beans, and potatoes. It kills most annual grasses and some annual broadleaf weeds as they germinate. It also provides inhibition of shoots from vegetative structures on some perennial grasses and sedges. Sutan (butylate) is also used in corn, but is not as effective on shattercane, johnsongrass and nutsedges as Eptam. This herbicide is widely used in several Midwestern states, primarily for annual grass control.

23 X Growth Regulators A. Introduction This group of herbicides includes four important groups; Benzoic acids, Carboxylic acids, Phenoxys, and Quinoline Carboxylic acids. These herbicides are labeled in many grass crops, primarily for control of broadleaf weeds, however, these herbicides do have significant grass activity, even to the point of injuring most labeled grass crops if the application rate or timing is not correct. These herbicides are labeled in corn, sorghum, wheat (and other small grains), pasture, rangeland, turfgrass, and rice. B. WSSA Herbicide MOA Classification (O) Synthetic auxins These herbicides differ in their soil activity, but most do have some soil activity. They are readily translocated throughout plants in both the symplast (phloem) and apoplast (xylem) systems. The exact sites of action of these herbicides are unknown. Researchers have determined that some of these herbicides interfere with cellular metabolism, RNA replication, mitosis, and other processes. Scientists have yet to determine the exact modes of action the herbicides inhibit to cause plant death, however, most agree that abnormal cell growth and division in and around the vascular tissues of plants (i.e. xylem and phloem) result in the collapse of these tissues, rendering the plant unable to transport water and nutrients through the xylem or sugars through the phloem. C. Herbicide Families Benzoic acids, Carboxylic acids, Phenoxy, and Quinoline carboxylic acids D. Symptoms The Benzoic acids, Carboxylic acids, and Phenoxy herbicides are widely used for control of broadleaf weeds in corn, sorghum, small grains, pasture, rangeland and turf. They are also used for woody plant control and in aquatics. The most obvious effects on susceptible plants are the twisting and downward curvature of the stems and leaves. Tissues that are growing rapidly are most susceptible to injury. If used at the wrong time, these herbicides can cause brittle stalks, fusing of brace roots, lodging, reduced tillering, and blasting of seed heads on grass crops. Except for the Quinoline Carboxylic acids, these herbicides kill broadleaf weeds in grass crops by translocating from the foliage to the growing points. They cause epinasty of broadleaf plants (Figure 1) and cupping of leaves. Plants may be twisted and deformed badly. In grass crops such as corn, if crop injury occurs, the brace roots may fuse together (Figure 2) and the shoots will buggy whip; (also called onion leaf). These herbicides are the main chemicals available that translocate well in broadleaf weeds and are very important for control of perennial broadleaf weeds such as field bindweed. They are also the main pasture herbicides. The Quinoline Carboxylic acid quinclorac (Paramount or Facet) is labeled in rice, wheat, and grain sorghum as a preemergence herbicide, and in pastures as Drive, as a foliar spray. As a preemergence material, quinclorac has activity on germinating grasses as well as germinating broadleaf weeds. It caused the same epinastic appearance in broadleaf plants and onion leafing of grasses as the other growth regulators.

24 Figure 1. Figure 2. E. Specific Herbicides 1. Benzoic acids Banvel or Clarity (dicamba) is readily translocated throughout plants in both the symplast and apoplast systems. This makes it useful for control of susceptible perennial broadleaf weeds such as field bindweed, milkweed, spurges and thistles. It will also control many woody species. It is used in corn, grain sorghum, pasture, rangeland, fallow between crops, wheat and turf. It can be hazardous around shrubs or in certain rotational crops because of its soil activity. Mixtures with 2,4-D are common. 2. Picolinic acids Tordon (picloram) is also sold in mixtures under other trade names [such as Grazon P+D (picloram + 2,4-D)]. It is the most persistent of the Carboxylic acids. It is used in pasture, rangeland, wheat and some other grass crops. Its use in wheat is very limited, except during the fallow period. It is excellent for perennial weed control, including bindweed and many brush species. Drift of even minute amounts can cause injury to susceptible crops, especially peanuts and soybeans. Lontrel (clopyralid) is a shorter residual Carboxylic acid than picloram and more selective. It is very effective on smartweeds, composites and legumes. It is also sold as Reclaim for mesquite control. Garlon or (triclopyr) differs slightly in its chemistry from the other Carboxylic acids, but otherwise is very similar. It does not last as long as picloram. It is used in non-cropland vegetation management, especially in perennial weed and brush control and in pastures and rangeland. 3. Phenoxys 2,4-D is the best known and widely used of the growth regulator herbicides. It is sold in different formulations and under different trade names. It is very important for the user to know the difference between amine and ester formulations of 2,4-D and their volatility potential. 2,4-D is used in small grains, corn, grain sorghum, pasture and rangeland, aquatics, non-cropland and in many turf weed control formulations of herbicides. It is often mixed with other herbicides because of a broader weed spectrum and a synergistic effect. It does not persist very long in soil. Applications should be made at crop growth stages before the reproductive growth is initiated. Early applications to wheat that has not fully tillered will result in reduced tillering and thus reduced yield.